When molten plastic is processed by an injection molding machine, the plastic enters a mold cavity where it is cooled to form a desired part shape. As the cooling occurs, the plastic contracts within the cavity. As a result of this contraction, the part actually shrinks in size, and sink marks or low spots often occur on the surface of the part. Shrink and sink marks have caused major problems for injection molders since injection molding was first developed. Several methods have been developed in an attempt to eliminate these problems. Some examples include gas-assisted injection molding, structural foam molding, liquid gas assisted molding, etc. In addition, foaming agents have been used in the molding process for mixing with molten plastic in order to generate inert gases in the plastic. These gases provide internal pressure in the plastic which enables the plastic to more fully fill the cavity of the mold and packs the plastic against the cavity walls. This, in turn, helps reduce sink on the surface of the plastic parts. Also, gas counterpressure in the mold cavity has been used to improve surface smoothness of molded parts.
These prior art methods are all problematic due to the large number of variables in the molding process. Varying injection pressures and injection speeds, varying melt pressures and temperatures, varying cavity conditions, and uncontrolled venting of gases all contribute to an unstable molding environment. These various problems in the molding process create burning and scission of polymer chains and create internal stresses within the plastic which remain in the plastic as the plastic material cools in the cavity. These internal stresses cause shrink, sink, and warpage of the plastic part to be molded. In addition, these various molding problems lead to degradation of the plastic material as it is processed through an injection molding machine. In general, erratic variations in pressure, temperature, and injection speed create material break-down and cause internal problems in the plastic which show up in the final molded product.
Another disadvantage of prior art systems is that the plastic melt flow in these systems experiences changes in pressure due to changes in cavity geometry as the molten plastic moves into the cavity of the mold. These pressure changes cause certain areas of the cavity to be filled more quickly than other areas resulting in different cooling characteristics in different areas of the cavity. These cooling variations cause inconsistencies in the direction of plastic solidification, which results in surface stresses, weld lines, or sink.
It is desirable to use a balanced injection molding process in which the pressure of the molten plastic is continuously controlled as the plastic moves through the injection molding machine. It is further desirable for an injection molding process to balance pressures acting upon the molten material in order to eliminate the above referenced problems caused by variations in polymer chain conditions so as to reduce internal stresses in the plastic. The ultimate goal of such an injection molding process is to produce a final product which nearly perfectly matches the cavity surface of the mold, is fully relieved of internal stresses which lead to shrink, sink, and warpage, and has greatly improved mechanical properties. In addition, part weight may be reduced, which will provide significant material savings to the manufacturers of such products.